The invention relates to a method for producing glass-ceramic parts and/or glass parts by means of deformation of a glass-ceramic blank and/or glass blank and to a device for carrying out said method.
Known methods of forming glass-ceramics, especially three-dimensional forming based on the state of the arts starts with a vitreous fabricated material because once the glass has been ceramized the only way of deforming it again is generally via the detour of melting.
In order to deform the basic glass of glass-ceramics with the forming methods usually used for glass, for example gravity lowering or vacuum lowering, it is typically heated to temperatures around 1000° C. where crystal growth will take place if nuclei were previously formed. When the basic glass is heated to the target temperature of 1000° C., for example, where crystal growth can occur, the nucleation range between 700° C. and 800° C., where minute crystallization nuclei are separated, inevitably has to be passed.
In order to prevent nucleation that may be inhomogeneous in the critical nucleation range and to prevent adverse affects to the properties of the glass-ceramic produced by the subsequent ceramizing process or prenucleation which will cause crystallization in the subsequent forming process, thereby making the process impossible, the nucleation range has to be passed as quickly as possible.
Forming glass originates with a glass blank and the usual deformation methods used for glass, such as gravity lowering or vacuum lowering, where the glass blank is typically heated to temperatures above the softening point of 1000° C., for example.
Heating the glass-ceramic blank or the glass blank quickly can be achieved, for example, by using high-capacity surface heaters.
Generally, the term surface heaters refers to heaters where at least 50% of the total heat flow from the heat source is supplied to the surface or the near-surface layers of the object to be heated.
A special type of surface heating a heating method using a gas flame where the flame temperatures are typically around 1000° C. Heating by means of a gas burner mostly takes place in that the heat energy of the hot gas is transferred across the surface of the glass-ceramic blank or glass blank. This may result in a temperature gradient that can adversely affect the forming because of viscosity gradients, for example. This particularly applies to glass thicknesses of ≧5 mm.
In order to through-heat the glass or glass-ceramic blank quickly by means of heat conduction the gas burner requires a high capacity output. Such heating is limited to small areas because gas burners are not capable of providing the required full-surface power density.
Therefore, heating with gas burners is especially not suitable for producing complex three dimensional glass-ceramics and is limited to simple geometries.
Other disadvantages of heating with gas burners are the following, for example:                relatively uncontrolled flame operation, and        intrusion of interfering gasses,which can adversely affect the quality of the material.        
Another option for producing three-dimensionally deformed glass-ceramics is placing them on the appropriate mold during the ceramizing process. However, the required low viscosities will not be obtained in this case, and as a result complex geometries can be formed only with very high bending radii.
PCT/FR96/00927 discloses reprocessing preliminary stages of glass-ceramics where the rolled ribbon of glass coming from high temperatures is formed directly at the melting tank when the required temperature is obtained, even before the critical nucleation range of glass-ceramics is reached.
The disadvantage of the method disclosed in PCT/FR96/00927 is the exceptionally high effort because the continuous process of forming glass has to be altered directly. Also, a subsequent forming process, separate from the tank operation, of intermediately stored glass-ceramic blanks for example, by means of reheating after they have cooled is not possible.
Another option for producing three-dimensionally formed glass is forming it in that it is positioned on the appropriate mold already during or after the melting process instead of using a glass blank.
In this case, the glass can be formed directly at the melting tank from a rolled ribbon of glass, for example.
The disadvantage of such a method is that the glass forming process is coupled to the tank operation.